Pertussis toxin-sensitive G protein mediation of PGE₂ inhibition of cAMP metabolism and phasic glucose-induced insulin secretion in HIT cells

Although prostaglandin E₂ (PGE₂) is known to inhibit glucose-induced insulin secretion, it is uncertain whether PGE₂ actions on the β-cell are direct, whether they are equipotent for both phases of hormone secretion, and whether the same mechanism of action prevails throughout. Study of the HIT cell, a clonal line of pancreatic β-cells, provides answers to these questions because perifusion with glucose and 3-isobutyl-1-methylxanthine stimulates biphasic insulin secretion. Perifusion with PGE₂ decreased both the first and second phases of glucose-induced insulin release to 47 ± 4% of controls. Pretreatment with pertussis toxin partly prevented PGE₂ inhibition to 80 ± 4% of controls for first phase and 79 ± 4% of controls for second phase. To evaluate whether the partial prevention of PGE₂ inhibition seen with pertussis toxin pretreatment was caused by G(i) heterotrimer association between the preincubation period and the end of perifusion, PGE₂ actions were also examined during continuous treatment with pertussis toxin. Under these conditions, PGE₂ inhibition of both phases was totally prevented. However, no difference was observed in membrane protein ADP ribosylation when cells were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after pretreatment or continuous treatment with pertussis toxin. Cyclic AMP (cAMP) accumulation was inhibited by PGE₂ (from 3263 ± 153 to 1549 ± 158 fmol/10⁶ cells) but less so after pretreatment with pertussis toxin (correlation between insulin release and cAMP accumulation during perifusion; n = 18, r = .85, P < .001). Thus, PGE₂ equally inhibits both phases of glucose-induced insulin secretion and cAMP generation through a pertussis toxin-sensitive G protein-mediated direct effect on the pancreatic β-cell.